Production of oil from well cased in permafrost

ABSTRACT

Thawing of permafrost surrounding a surface casing of a wellbore is minimized during production of hot oil by injecting liquid refrigerant in an annulus between the surface casing and the production casing, allowing heat from the hot oil to vaporize the refrigerant as it descends in said annulus, and removing the resulting vaporized refrigerant at the well-head, thereby maintaining the surface casing at a temperature of 32* F. or lower.

United States Patent 1191 Willhite 1 Oct. 23, 1973 i 1 PRODUCTION OF OILFROM WELL CASED 1N PERMAFROST [75] Inventor: Glen Paul Willhite,Lawrence, Kans.

[73] Assignee: The Kansas University Endowment Association, Lawrence,Kans.

[22] Filed: Dec. 1, 1971 [21] App]. No.: 203,605

[52] US. Cl. 166/302, 166/DIG. 1 [51] Int. Cl E2lb 43/24 [58] Field ofSearch 166/302, DIG. l,

[56] References Cited UNITED, STATES PATENTS 5/1972 Keeler et al.166/302 10 1971 Hyde et a1. 166/315 3,703,929 11/1972 Rardin 166/3023,642,065 2/1972 Blount 166/244 R 3,217,791 11/1965 Long 165/45 PrimaryExaminerStephen J. Novosad Att0rneyWilliam G. Ewert [57] ABSTRACTThawing of permafrost surrounding a surface casing of a wellbore isminimized during production of hot oil by injecting liquid refrigerantin an annulus between the surface casing and the production casing,allowing heat from the hot oil to vaporize the refrigerant as itdescends in said annulus, and removing the resulting vaporizedrefrigerant at the well-head, thereby maintaining the surface casing ata temperature of 32 F. or lower.

PATENIEllum 23 ms SHEET 2 BF 2 l/vvE/v T01? GLEN PAUL W/LLH/TE BATTORNEY PRODUCTION OF OIL FROM WELL CASED IN PERMAFROST This inventionrelates to the production of oil. In another aspect it relates to amethod and means for the production of oil from a well cased inpermafrost while minimizing the thawing of the permafrost.

Several years ago, a major oil discovery was made in an arctic region inAlaska now known as the North Slope. The oil reservoirs of thisdiscovery are located at depths ranging from 8,000 to 10,000 feet. Theremote region in which this discovery was made is an unusual environmentfor well completion in that permanently frozen earth (or permafrost),comprising a frozen non-uniform aggregation of ice, sand, gravel, orsediment, extends over the region with thicknesses up to 2,300 feet, theupper 50200 feet often containing a high percentage of ice (e.g. 40percent), the average temperature of the permafrost being about 16 F.and increasing linearly with depth at a rate of about 0.016 F. per footfrom -15" F. at the surface. The loadbearing strength of the permafrostdepends upon the extent it is frozen.

Oil wells are drilled through the permafrost to deeper oil-bearingstrata or reservoirs to produce oil at high rates. The temperatures ofthe reservoirs are in the vicinity of 150 F. to 200 F. and the highrates of production (e.g., 2,000 to 15,000 barrels per day) generallykeep the temperature of the produced fluids from decreasingsignificantly as they ascend from the reservoir to the wellhead. Theproduced oil, being significantly higher in temperature than thepermafrost surrounding the well casing, causes thawing or melting aroundthe wellbore and destruction of the cement bond at the cement-permafrostinterface. The resulting thawed permafrost shrinks and subsides and thedowndrag force or movementof the thawed permafrost exerts a frictionalforce or mechanical stress on the surface casing, for example a force of2,000 lbs./ft down to about 500 ft. This downdrag can damage the wellcasing (e.g.causing crumpling, twisting, or collapse thereof) andnecessitate premature abandonment of the well and the subsidenceresulting from an extensive thawed zone (e.g. with a radius of up to 20feet) will induce soil erosion when spring thaw or rains come.

Several solutions have been proposed to prevent thawing of permafrostduring oil production (see The Oil and Gas Journal, Dec. 8, 1969, page69 and Paper No. 325 1 (1971) of the Society of Petroleum Engineers ofAIME). Such solutions include the use of insulation (polyurethane) .tolimit the thaw radius or keep the upper portion of the permafrost fromthawing and the circulation of a cooling liquid down the hole and backto maintain temperatures below 32 F. The art also discloses a number oftechniques for controlling the temperature of a well in connection withconventional production of oil (see U.S. Pat. Nos. 3,004,601, 3,013,609,3,259,185, 3,433,641, and 3,456,734). But the heretofore proposedsolutions to the thawing permafrost problem and the techniques usedinconventional production are limited and have several disadvantageswhich make them impractical or inefficient, insofar as minimizing thethawing of permafrost is concerned.

Briefly, according to this invention, the thawing of permafrostsurrounding a surface casing of a wellbore is minimized to a desireddegree by continuously injecting subcooled liquid refrigerant into theupper end of an annulus extending through all or the upper portion ofthe permafrost interval between the surface casing and the production orcompletion string disposed within the surface casing (the stringcomprising a production casing and/or one or more production tubings),allowing the heat from the produced fluid flowing through the wellboreto vaporize the refrigerant as it descends in said annulus, andcontinuously removing the resulting vaporized refrigerant from thewellhead, thereby maintaining the temperature from the casing, adjacentthe locus of vaporization, at about 32 F. or lower.

The injection of the liquid refrigerant and its vaporization can becarried out in a number of different ways. The injected liquidrefrigerant can be allowed to descend in said annulus as a falling filmon the inner wall of the surface casing, the heat from produced fluid(oil and/or gas) flowing through the production string (which can beencircled by insulating medium) causing the vaporization of the fallingfilm over the course of its descent. Alternatively, a perforated orslotted pipe can be disposed in an annulus between the surface casingand production string, and liquid refrigerant can be injected into theupper end of the perforated pipe with each of the perforations in thepipe functioning in effect as an expansion valve for the vaporization ofthe liquid refrigerant in the annulus. Another way of accomplishing thetemperature control of this invention is to hang or dispose within saidannulus a perforated or slotted casing or sleeve so as to divide theannulus into an outer annular region and an inner angular region, withcommunication between the two regions being had by the perforations inthe perforated casing, such perforations also acting in effect asexpansion valves; liquid refrigerant is introduced into the upper end ofthe outer annular region, filling the outer annular region, and theliquid refrigerant is passed through the perforations and vaporizes intothe inner annular region. The rates of injection of the liquidrefrigerant by these various techniques, as well as the pressuremaintained in the annulus, are chosen so as to prevent or minimize theaccumulation of the liquid refrigerant at the lower terminus of theannulus, and the method of operating and means employed are such as toprevent any commingling of the liquid or vaporized refrigerant with anyproduction fluids. The vaporized refrigerant removed at the wellhead canbe compressed and liquefied and recycled to the annulus.

In the accompanying drawing, the various figures schematicallyillustrate in elevation and partial section wells cased in permafrostprovided with means in accordance with this invention to minimize thethawing of the permafrost. In these various figures, the same referencenumbers have been used to designate like parts.

Referring to the embodiment shown in FIG. 1, a surface casing (orpermafrost casing") 1 can penetrate part or, as shown, all of permafroststratum or interval 2 and is cased or cemented 3 thereto, preferably, asshown, from its lower end to the surface. Disposed within the surfacecasing l is a production casing 4, the lower portion of the productioncasing depending below surface casing 1 and the lower boundary 5 orbottom of the permafrost. Production casing is cemented 6 to the strata7 below the permafrost stratum, an annular space or annulus 8 beingformed between the two casings 1,4 with the lower end of the annulusbeing closed by cement 6 or the like (such as a packer).

Though the lower end of annulus 8 is shown sealed by cement at a depthcoincident with the bottom of the permafrost zone 5, it can be sealed ata point thereabove (eg with cement or a suitable packer) so as to keeponly the upper portion of the surrounding permafrost zone frozen, e.g.,to a depth of 500 to 1,000 feet, since it is the upper portion of thepermafrost zone where the most severe thawing is normally encounteredduring production. One or several producting tubings, such asillustrated by reference number 9, can depend within the productioncasing 4 forming an annulus 11. (The outer wall of the productionstring, such as production tubing 9, can be encased with or encircled byone or several layers of insulation as described below.) Suitableconventional exit pipes 12, 13 are connected to the upper ends of theproduction casing 4 and tubing 9. For example, surface casing 1 can havea diameter of 20 inches and can be set in a 26 inch hole at a depth of2,300 feet; production casing 4 can have a diameter of 13 inches and canbe set at a depth of 8,000 feet; and the production tubing 9 can have adiameter of 7 inches. The production casing and tubing extend to the oil(and/or gas) -bearing strata but such has been omitted in the interestof brevity together with the conventional appurtenances used in aproducing well, such as a surface conductor, casinghead collar, etc.

The liquid refrigerant used in this invention, for example sub-cooledliquefied propane, can be supplied by line 14 and injected via opening16 at the upper end of annulus 8. An annular deflection plate or collar18 can be hung from the upper end of casing 1 at the locus of injectionso as to distribute the liquid refrigerant around the upper end ofannulus 8 and direct it to the inner wall of the surface casing l. Theinjected liquid refrigerant descends along the inner wall of surfacecasing 1 in the form of a falling film, the heat in the wellbore, suchas that emanating from hot oil being produced by production tube 9and/or annulus 11 in the production casing 4, causing vaporization ofthe falling film as it descends. In order to ensure substantiallyuniform or continuous film or liquid refrigerant on the inner wall ofsurface casing l, and avoid hot spots due to non-uniform liquidvaporization, a plurality of annular distributor or redistributor platescan be affixed within annulus 8 to surface casing 1 or production casing4, for example at the collars joining the casing sections (each sectionbeing for example 30-35 feet in length). Other means will be obvious tothose skilled in the art for ensuring uniform liquid distribution forthe refrigerant as it courses down the inner wall of the surface casing.The vaporized refrigerant flowing upward and countercurrent to the flowof liquid refrigerant, can be removed at the upper end of annulus 8 viaopening 19 in surface casing l and conveyed via pipe 21 to a compressoror pump 22, such as used in mechanical refrigeration systems, and thecompressed gas cooled by heat exchanger 23 and passed via line 24 to asecond heat exchanger 26 where it is liquefied, the liquefiedrefrigerant 27 then being recycled to supply line 14.

The rate of injection of the liquid refrigerant can be controlled toretain free gas space within the annulus 8. The injection rate should becontrolled to prevent or minimize the accumulation of any liquidrefrigerant at the lower terminus of annulus 8. Such accumulation shouldbe avoided since even a small head of liquid would increase thesaturation temperature, i.e., the temperature at which vaporizationbegins, and consequently increase the temperature of the permafrost atthat depth, causing thawing thereof. Stated otherwise, the rate ofinjection is controlled so as to prevent any accumulation of any liquidrefrigerant in the annulus, since a liquid-filled annulus would notprevent thawing of the permafrost at the locus of accumulation unlessthe boiling point of the liquid is less than 32 F. at the maximumpressure exerted by the head of accumulated liquid refrigerant.

The drag force of the ascending vaporized refrigerant in annulus 8 onthe descending liquid film will be at its maximum near the surface wherethe mass flow rate of the vaporized refrigerant is at its maximum.However, this drag force will not be significant or a limiting factor onthe flow rate of the liquid refrigerant necessary to ensuresubstantially complete coverage of the inner wall of surface casing lwith a falling film of liquid refrigerant The heat from the hot producedoil in production tubing 9 will flow by conduction, natural convection,and radiation to the production casing 4 and heat from the latter willflow by forced convection and radiation into the annulus 8 to thesurface of the falling film of liquid refrigerant, causing vaporizationof the latter at or below the surface of the falling film. The injectionrate of the liquid refrigerant can be determined by equating the rate ofheat flow from the hot oil to the rate of heat absorbed by vaporization,the total rate of heat flow being proportional to depth. The maximumdepth at which vaporization can be controlled within the annulus will bedetermined by such factors as the rate of heat flow from productiontubing 9 to production casing 4, the extent that the inner wall of thesurface casing is covered by the falling film, the free gas space in theannulus, and the downward liquid flow of the refrigerant. The force ofgravity on the descending liquid will exceed the friction between thefalling film and the surface casing as well as exceed the drag forceexerted by the rising vaporized refrigerant or gas at the gas-liquidinterface, which would entrain some of the liquid. Thus, for moreefficient operation, the falling film of liquid refrigerant should bedistributed over a substantial portion of the inner wall of the surfacecasing. The vaporization temperature of the refrigerant will bedetermined by the thermodynamic properties of the liquid refrigerant andthe pressure that is maintained in the annulus. In order to keep thepermafrost from melting, the vaporization temperature should b 32 F. orless.

Ammonia, carbon dioxide, and propane have suitable thermodynamicproperties for use as a liquid refrigerant. Propane will be apreferred'refrigerant because of its desired properties and because itprobably will be available at the well-site where it can be obtainedfrom the gas produced with the crude oil. In the Table I below, thesaturation temperatures and pressures for propane are given.

TABLE I Thermodynamic Data for Propane Temp. Sat. Press. Temp. Sat.Press.

p.s.i.a. F. p.s.i.a. l0 30.95 40 77.80 0 37.81 50 91.50 10 45.85 60106.90 20 55.00 124.30 30 65.70 143.60

At 30 F., the saturation pressure of propane is 65.7 psia. This shouldbe ample pressure to push the vaporized propane out the annulus at theanticipated vaporization rates.

Calculations were made to determine surface refrigeration capacity andthe rate of flow of liquid refrigerant (propane) necessary to maintainthe surface casing 1 of FIG. 1 at a temperature of 32 F. for variouspermafrost thicknesses (i.e., the rate necessary to prevent any heatflowing from the surface casing to the surrounding permafrost 2 atcertain thicknesses of the permafrost). In these calculations, thesurface casing 1 was assumed to have a diameter of 16 inches andcemented through the permafrost interval. The production casing 4 wasassumed to be 13 in diameter (and was not covered by an insulationlayer) and the production tubing 9 was assumed to be 7 inch in diameterwith oil flowing therethrough at 180 F. The permafrost temperature wasassumed to be 32 F. through the entire permafrost interval. The resultsof these calculations are presented below in Table II, with 200 ft.deducted to approximate the effect of vertical heat flow.

TABLE [1 Rate of Surface Depth of liquid refrigeration permafrostrefrigerant, capacity maintained at 32F. lbsJhr. BTU/hr. Ft. 1250203,400 512 1500 244,000 629 1750 284,000 744.6 2000 325,000 858 2250366,000 970 2500 407,000 1081 2750 447,500 1191 3000 488,000 1300 3250528,000 1408 3500 569,000 1516 3750 610,000 1623 4000 650,000 1730 4250691,000 1836 4500 731,000 1941 The data of Table 11 show that permafrostthawing can be prevented at all depths by controlling the injection rateof the liquid refrigerant.

As shown in FIG.2, the outer surface of production tubing 9 can beadhesively bonded to a layer 40 of insulation material such as a 1 to33-inch thick layer of polyurethane foam (having a density of 2-6lbs/ft), which can extend over only the upper portion of the permafrost(e.g. 500l,000 ft.) or extend down as low as the lower boundary 5 of thepermafrost, or lower. Such insulation can be covered on its exterior orwrapped with an aluminized Mylar film, or the like, to serve as abarrier to heat radiation. Such embodiment will permit the use of lowerrefrigerant rates, and thus decrease the operating costs. The liquidrefrigerat introduced into the annulus 8 can be allowed to fall as afilm on the interior of the surface casing l or as a film on the exposedsurface of the insulation. Calculations were made for this embodimentlike those given above in Table II, assuming a surface casing having adiameter of 13 inches and a production tubing having a diameter of 7inches and covered with a 1 inch layer of polyurethane foam insulation(density 4 lbs/ft, thermal conductivity 0.015 BTU/hr. ft. F.) extendingthe entire length of the permafrost stratum, the falling film of liquidrefrigerant (either propane or ammonia) descending on the inner wall ofthe surface casing. Refrigeration requirements necessary to keep variousthicknesses of permafrost from thawing are set forth in Table III.Again, the

temperature of the permafrost interval was assumed to be 32 F.

TABLE III Rate of Surface Depth of liquid refrigeration permafrostrefrigerant capacity, maintained lbs/hr. B U/hr. at 32 F ft. Propane 25040,700 659 350 56,900 1067 450 73,200 1471 550 89,500 1872 650 106,0002272 Ammonia The data of Table 111 show that refrigerant rates are quitelow, compared to those of Table II, and thus the operating costs shouldalso be low. Similar results were obtained when the surface casingdiameters were 16 inches and 20 inches.

Referring now to FIG. 3, instead of using a layer of solid insulationmaterial to lower the refrigeration requirements, two further strings ofcasing 20,25 can be disposed within annulus 8, so as to form an innerannulus 35 and an outer annulus 30, which communicate at their lowerends and are filled with a non-circulating insulating liquid, such asgelled oil, or the like. The insulating liquid can be placed by pumpingit down the outer annulus 30 and returning it via the inner annulus 35,suitable supply and withdrawal pipes being connected to the upper endsof these annuli, as shown, for placement purposes. In such embodiment,the refrigerating liquid can be allowed to descend on the inner wall ofthe surface casing 1 or the outer wall of easing string 20. Calculationswere made like those given above in Table II, assuming a surface casinghaving a diameter of 20 inches, insulating fluid placement casingshaving diameters of 13 36 inches and 9 inches, and a production stringhaving a diameter of 7 inches, and using gelled oil as the insulatingliquid. Refrigeration requirements for this embodiment are shown inTable IV, using propane as the liquid refrigerant. The permafrosttemperature was assumed to be 32 F.

TABLE IV Rate of Surface Depth of liquid refrigerant permafrostrefrigerant, capacity, at 32F., 1bs.lhr. BTU/hr. ft. 400 65,000 377 60097,500 660 800 130,000 937 1000 162,800 1210 1200 195,000 1479 1400228,000 1745 1600 260.000 2009 1800 292,000 2270 Referring now to FIG.4, another embodiment of this invention is shown. In this embodiment, aperforated or slotted casing or sleeve 31 is mounted within the annulus8 between surface casing 1 and production casing 4 and depends thereinforming an outer annular region 32 and an inner annular region 33, thelower ends of which are sealed. Slotted casing 31 is provided with aplurality of slots or perforations 34 along its length. Liquidrefrigerant is supplied via pipe 14 and opening 16 to the upper end ofthe outer annular region 32 at a temperature below 32 F. and fills thatregion. The liquid refrigerant is forced from the annular region 32 viaslots 34 into the inner annular region 33, a portion of the liquidrefrigerant vaporizing as it enters the latter and a portion of itdescending along the inner wall 45 of the slotted casing in the form ofa falling film in a manner described in connection with the embodimentshown in FIG. 1. The heat flow from the production casing causesvaporization of this falling film as it descends. The temperature of theslotted casing 31' is controlled at 32 F. or less by the pressure inannulus 32 between the production casing 4 and the surface of thefalling film of the refrigerant on the inner wall 35 of the slottedcasing. The flow of the falling film of liquid is controlled by thepressure in the outer and inner annular regions 32, 33, the size of theslots or openings 34, and the distribution of the slots with respect todepth. Since the pressure in the liquid-filled annulus 32 will increasewith depth, the size or the number of slots 34 should decrease withdepth. In lieu of mere openings or slots in casing 31, those skilled inthe art will recognize that other equivalent means can be used, such asspray nozzles or porous metal discs.

Referring now to the embodiment shown in FIG. 5, a perforated string oftubing or pipe 36 is disposed within annulus 8. Liquefied refrigerant issupplied via line 14 to the upper end of perforated tubing 36, the holesor openings 37 therein being used to spray the liquid refrigerant on theinner wall of surface casing l and the outer wall of production casing4, vaporization of the liquid refrigerant occuring on these walls, thusremoving heat emanating from the production casing, such heat beingremoved as latent heat in the vaporized refrigerant as it is removed vialine 21 from the upper end of annulus 8. The pressure in annulus 8 isregulated so that the vaporization temperature is 32 F. less. In orderto operate at temperatures lower than 32 F., and thereby keep the entiresurface casing temperature at a temperature less than 32 F., it may bedesirable to disposewithin annulus 8 a plurality of such perforatedtubings.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention.

What is claimed is: u

1. A method of minimizing the thawing of permafrost surrounding awellbore in which are disposed a surface casing'adjacent said permafrostand production string disposed within said surface casing with anannulus between said casing and said production string, which comprises,during production, injecting liquefied refrigerant into said annulus,passing said injected refrigerant downwardly in said annulus as afalling film on the inner wall of said surface casing while retainingfree gas space in said annulus,'allowing the heat emanating from saidproduction string to vaporize said falling film of said injectedrefrigerant as it descends on said inner wall over a finite distance,and removing the resulting vaporized refrigerant from the upper end ofsaid annulus without coinmingling the same with production fluid andwithout accumulating liquid refrigerant within said annulus. v

2. The method according to claim 1, wherein said production string isencircled by insulation medium, such as foamed polyurethane or gelledoil.

3. The method according to claim 1, wherein said liquefied refrigerantis propane or ammonia.

production string comprises a production casing which can have one ormore production tubings disposed therein, and wherein said annulus isbetween said surface casing and said production casing.

6. The method according to claim 1, wherein said production stringcomprises a'production tubing, the

outer surface of which is bonded to a layer .of foamed polyurethaneinsulation extending over the upper portion of all of said permafrost. m7

7. Apparatus for producing fluids from reservoirs penetrated by awellbore surrounded at its upper end by permafrost, which comprisessurface casing and production string depending within said surfacecasing with an annulus therebetween, means to inject liquefiedrefrigerant in said annulus, means to direct said injected liquefiedrefrigerant to the inner wall of said surface casing to cause saidinjected refrigerant to descend thereon as a falling film, and means forwithdrawing vaporized refrigerant from the upper end of said annulus. 8.Apparatus according to claim 7, further compris-' ing insulation means,such as polyurethane foam or gelled oil, encircling said productionstring. 9. Apparatus according to claim 7, wherein said productionstring comprises a production casing which can have one or moreproduction tubings disposed therein, said annulus being between saidsurface casing and said production casing.

10. Apparatus according to claim 7, further comprising a perforatedsleeve depending within said annulus and dividing the same into anouterannular region and an inner annular region, saidmeans for injectingcommunicating with the upper end of said outer annular region, and saidmeans forwithdrawing vaporized refrig-- erant communicating with saidinner annular region.

11. Apparatus according to claim 7,further comprising perforated tubingdepending within said annulus, said means for injecting communicatingwith the upper end of said perforated tubing.

12. Apparatus according to claim 7 ,further comprising two concentricinsulating placement casings encircling said production string with anouter annular region between the said placementcasings and an innerannular region between said production string and the innermost of saidplacement casings, said annular regisiireihg'aika was insulation fluid,said annulus in which said liquid refrigerant is injected beingbetweensaid surface casing and the outermost of said placement casings.

13. Apparatus according to claim 7 ,further comprising means forcompressing and liquefying said withdrawn vaporized refrigerant andmeans for recycling the resulting reliquefied refrigerant tosaid meansfor injecting.

14. Apparatus according to claim 7 wherein said pro si ma tan comprisesa p agsaaaruismgravrng' a layer of foanied polyurethane insulaTionbohTiedtFtHei outer surface of said production tubing and extendingavrme'up 'er portion or all of j ifA meth odof minimizing the thawing ofperma frost surrounding a wellbore in which are disposed a surfacecasing adjacent said permafrost and producforations and a portion of itpasses downwardly in said inner annular region as a falling film on theinner wall of said perforated sleeve while retaining free gas space insaid inner annular region, heat emanating from said production stringvaporizing said falling film as it descends within said inner annularregion. the resulting vaporized refrigerant ascending in said innerannular region and being removed from the upper end thereof withoutcommingling the same with production fluid and without accumulatingliquid refrigerant within said inner annular region.

2. The method according to claim 1, wherein said production string isencircled by insulation medium, such as foamed polyurethane or gelledoil.
 3. The method according to claim 1, wherein said liquefiedrefrigerant is propane or ammonia.
 4. The method according to claim 1,wherein said injected liquefied refrigerant is injected at a ratesufficient to ensure substantially complete coverage of said inner wallof said surface casing with said falling film of refrigerant.
 5. Themethod according to claim 4, wherein said production string comprises aproduction casing which can have one or more production tubings disposedtherein, and wherein said annulus is between said surface casing andsaid production casing.
 6. The method according to claim 1, wherein saidproduction string comprises a production tubing, the outer surface ofwhich is bonded to a layer of foamed polyurethane insulation extendingover the upper portion of all of said permafrost.
 7. Apparatus forproducing fluids from reservoirs penetrated by a wellbore surrounded atits upper end by permafrost, which comprises surface casing andproduction string depending within said surface casing with an annulustherebetween, means to inject liquefied refrigerant in said annulus,means to direct said injected liquefied refrigerant to the inner wall ofsaid surface casing to cause said injected refrigerant to descendthereon as a falling film, and means for withdrawing vaporizedrefrigerant from the upper end of said annulus.
 8. Apparatus accordingto claim 7, further comprising insulation means, such as polyurethanefoam or gelled oil, encircling said production string.
 9. Apparatusaccording to claim 7, wherein said production string comprises aproduction casing which can have one or more production tubings disposedtherein, said annulus being between said surface casing and saidproduction casing.
 10. Apparatus according to claim 7, furthercomprising a perforated sleeve depending within said annulus anddividing the same into an outer annular region and an inner annularregion, said means for injecting communicating with the upper end ofsaid outer annular region, and said means for withdrawing vaporizedrefrigerant communicating with said inner annular region.
 11. Apparatusaccording to claim 7, further comprising perforated tubing dependingwithin said annulus, said means for injecting communicating with theupper end of said perforated tubing.
 12. ApparatuS according to claim 7,further comprising two concentric insulating placement casingsencircling said production string with an outer annular region betweenthe said placement casings and an inner annular region between saidproduction string and the innermost of said placement casings, saidannular regions being filled with insulation fluid, said annulus inwhich said liquid refrigerant is injected being between said surfacecasing and the outermost of said placement casings.
 13. Apparatusaccording to claim 7, further comprising means for compressing andliquefying said withdrawn vaporized refrigerant and means for recyclingthe resulting reliquefied refrigerant to said means for injecting. 14.Apparatus according to claim 7 wherein said production string comprisesa production tubing having a layer of foamed polyurethane insulationbonded to the outer surface of said production tubing and extending overthe upper portion or all of said permafrost.
 15. A method of minimizingthe thawing of permafrost surrounding a wellbore in which are disposed asurface casing adjacent said permafrost and production string disposedwithin said surface casing with an annulus between said casing and saidproduction string, wherein said annulus is divided into an inner annularregion and an outer annular region by means of a perforated sleevedepending within said annulus, which method comprises, duringproduction, injecting liquefied refrigerant into the upper end of saidouter annular region, filling the same, passing said liquefiedrefrigerant from said outer annular region into said inner annularregion through perforations in said perforated sleeve whereby a portionof said liquid refrigerant vaporizes as it passes through saidperforations and a portion of it passes downwardly in said inner annularregion as a falling film on the inner wall of said perforated sleevewhile retaining free gas space in said inner annular region, heatemanating from said production string vaporizing said falling film as itdescends within said inner annular region, the resulting vaporizedrefrigerant ascending in said inner annular region and being removedfrom the upper end thereof without commingling the same with productionfluid and without accumulating liquid refrigerant within said innerannular region.